Antennas
5.6 CIRCULAR POLARIZATION
T-R
T-R
T-R
T-R
Power divider to give cosecant squared transmitted beam
From the transmitter
Receiving beam 1 to receiver 1 Receiving beam 2 to receiver 2 Receiving beam 3 to receiver 3 Receiving beam 4 to receiver 4 Stack of
horns
Figure 5.60 The arrangement of horns and transmit-receive switches to give a single transmitted beam and a number of stacked receiving beams.
Tapered transition to a symmetrically orthogonal form,
either square or circular
Circular polarizer
Output waveguide, square or circular
Symmetrical radiator
Normal rectangular waveguide
Optional orthogonal
port
Slat depth, l ' 38g
4 or 8g
4 according to the sense of polarization
Slat separation, a ' 0.671 8g Figure 5.61 A generic waveguide polarizer for feed horns.
(5.89)
(5.90) higher reactance in the middle [19]. The gradual transition reduces the standing waves and increases the bandwidth. The total reactance of the irises is chosen to give the orthogonal waves exactly 90 degrees phase difference. The waveguide containing the irises may be rotated through 45 degrees to be in line with the input and output waveguides. Although there is a phase shift in the main mode, there is no transition to circular polarization. The linear polarization mode is used in clear conditions to obtain maximum radar range.
Other types of circular polarizer use diagonal posts placed a quarter of a wavelength apart or dielectric vanes, which have arrow-shaped ends.
If the waves in the waveguide in Figure 5.62 are horizontally polarized, the delay occurs in the opposite order and right circular polarization is produced. The coupler shown dotted in Figure 5.61 couples the horizontal component into or out of the polarizer, which is used to extract the rain clutter signal for a rain or weather display in airport radars.
5.6.2 Reflecting polarizers
Reflecting polarizers are in the form of diagonal grids in reflectors [20, p. 23-26]. Such a polarizer is shown in Figure 5.63. Two diagonal orthogonal grids are placed an eighth of a wavelength apart, so there is an extra path length of a quarter wavelength or 90 degrees in the phase of one component. The example shown produces left circular polarization when looking in the direction of propagation. Note that illumination from a horizontally polarized wave would produce right circular polarization.
5.6.3 Transmission polarizers
Parallel plate, dielectric, or meander transmission polarizers [20, p. 23-26] are placed in front of the antenna or feed horn system and one is shown in Figure 5.64. The reference gives the following equations for the values of a and l:
Reactive iris
Reactive post Dielectric
blade C
L
Resolve the vertical voltage field into two 45 degree components. The vector
marked C sees capacitance and is advanced in phase, the vector marked L sees inductance and is retarded in phase.
C
L
L gives delay Initial TE11 or H11
vertical electrical field in a circular waveguide
Direction of propagation
The resulting left circular polarization As the electrical field moves forwards
the rotating electric field vector moves to the left.
L gives delay
Guide wavelength, 8g ' 80 1 & 80
2a
2
Figure 5.62 The principle of circular polarizers using irises, posts, and blades.
(5.91)
where 80 is the free space wavelength.
The slats are moved mechanically either in front of the feed system or to storage at the side.
5.6.4 Phased array polarization
Phased arrays may be designed for circular polarization either with an external polarizer (slats) or with each discrete element radiating in a circularly polarized mode. Such elements are quadrature fed crossed dipoles, axial mode helices, or circularly polarized patches. To switch between the types of polarization, each element must be switched to radiate in a different mode.
Resolve the vertical voltage field into two 45 degree components. The vector tilted to the left sees the first reflecting grid and is reflected,
the vector tilted to the right sees the second grid and is retarded by 90 degrees in phase.
Initial vertical electrical field Direction of propagation
The resulting left circular polarization As the electrical field
moves forwards the rotating electric field vector moves to the left.
Direction of propagation
8 8
Two orthogonal grids each reflecting one of the
diagonal components
Resolve the vertical voltage field into two 45 degree components. The vector tilted to the right sees the slats and is advanced by 90 degrees in phase. The
vector tilted to the left is unaffected.
Initial vertical electrical field
The resulting left circular polarization As the electrical field moves forwards the
rotating electric field vector moves to the left.
Direction of propagation
a l
The slats are spaced at a m and have a depth of l m.
Figure 5.63 Principle of a reflector polarizer.
Figure 5.64 Circular polarization formed by diagonal slats.
ellipticity, eell ' a
b or 20 log10 a b dB
Trace from antenna measurement
Peak = a Trough = b
Zero line
, ' 1 & b²
a²
(5.92)
Figure 5.65 The wavy antenna characteristic trace with a rotating transmitting dipole.
5.6.5 Engineers’ and physicists’ conventions
There is a contrast as to how engineers and physicists define polarization, as shown in Table 5.5.
Table 5.5
Engineers’ and physicists’ conventions for circular polarization
Electrical engineering Physics
Linear polarization Linear polarization
The direction of linear polarization is given by the The definition is not so clear: older literature uses the direction of the electric field normal to the direction of direction of the magnetic field vector and newer propagation (see IEEE Standard 100). literature the electric field vector [21, p. 6-4].
Circular polarization Circular polarization
The sense is given by the direction of rotation of the This is given by the rotation of the electric field vector electric field vector with time as the wave passes with time as the wave passes through a plane normal to through a plane normal to the direction of propagation the direction of propagation as seen by an observer as seen by an observer at the source (see IEEE looking towards the source [21, p. 6-4].
Standard 100).
The different conventions caused problems and delays with the first transmissions using satellites between the United States and England.
5.6.6 Ellipticity or the quality of circular polarization
The quality of circular polarization is measured by using a rotating dipole as a transmitting antenna during antenna testing. If the polarization of the antenna on test is perfectly circular, it will deliver a constant signal. Otherwise, the signal will be modulated by the transmitting antenna rotation, as in Figure 5.65. If the peaks are a voltage units and the troughs b voltage units, the ellipticity is given by
Note that eccentricity is defined as
It is customary to measure the ellipticity, e , over the whole of the main beam, at least between the 10 dB points.ell The average used for finding the integrated cancellation ratio (ICR) is weighted by the two-way antenna voltage gain in that direction.
Measure of suppression ' eell2 % 1 eell2 & 1
ratio or 20 log10 eell2 % 1 eell2 & 1
dB
10 12 14 16 18 20 22 24 26 28 30
0 0.5 1 1.5 2 2.5 3
Weighted mean ellipticity, dB
(5.93)
Figure 5.66 The effects of ellipticity on rain echo reduction.
5.6.7 Rain echo suppression
The amount of rain echo reduction [17, p. 7.6] is given by the measure of suppression with e as the average ellipticityell
If (5.93) is weighted by the two-way antenna pattern and integrated, it is called the integrated cancellation ratio, where e is the average ellipticity weighted by the two-way antenna voltage characteristic.ell
Figure 5.66 shows this in graphical form.